Stress induced increases in hormones are thought to be involved in the disruptive effects of stress on reproduction. Acute and chronic stress results in an elevation of prolactin and elevated prolactin is associated with some cases of premenstrual syndrome, corpus luteum insufficiency, and infertility. Vitex, a natural plant product, has been shown to be clinically effective in alleviating symptoms in conditions associated with disorders of the menstrual cycle when prolactin is elevated, but the mechanism by which it acts is not well understood. The goal of this research is to further describe the mechanism of action of Vitex. Female sheep will be given an acute physiological stress (bacterial endotoxin) during the follicular phase in a manner known to disrupt the subsequent phases of their reproductive cycle. In the initial study, animals will be treated with Vitex to determine if Vitex will inhibit the stress induced release of prolactin and reduce the severity of the stress induced defects in hormonal levels and estrus cycle length. In a subsequent study the mechanism by which Vitex lowers prolactin will be evaluated by administering Vitex alone or concurrently with D1 or D2 antagonists in sheep during the non-breeding season. Finally, the effects of Vitex will be compared to the effects of D1 and D2 agonists and Vitex with either a D1 or D2 antagonists on stress induced defects in an experimental design similar to the first study.